Polysulfone resin films are generally manufactured by means of the hot-melt extrusion method and the solution casting method. These films are known to be superior in optical properties, mechanical strength properties, electrical properties, transparency, heat resistance, flame resistance, etc. Because of these superior properties, this film is, for example, stretched and used as an optical filter such as a phase retarder for a liquid crystal display device.
When manufacturing a film by the solution casting method, the solution viscosity is relatively low, and therefore it is possible to remove foreign matter by using fine mesh filters. As a result, this film develops very few fish-eyes (defects in transparent or half-transparent plastic films or sheets which look like small spherical lumps; one of the reasons why they are generated is that mixing is not sufficient between those lumps and the surrounding resin), and tends to be superior in appearance, surface smoothness, transparency and thickness accuracy.
When manufacturing a polysulfone resin film by the aforementioned solution casting method, a methylene chloride solution of polysulfone resin is widely used as a resin casing solution.
For example, a polysulfone resin film is obtained by casting a methylene chloride solution of this resin onto a support piece using an appropriate coater, evaporating the solvent, and peeling the film-like product thus formed from the support piece.
This manufacturing method is superior in that said methylene chloride is non-inflammable, has a low boiling point (approximately 40.degree. C.), and is a polar solvent with a high drying efficiency. However, when the polysulfone resin is dissolved in methylene chloride, ringed polymers and low molecular weight components which are byproducts of the synthesis of the polysulfone resin gradually aggregate and crystallize in the solution and become non-soluble in methylene chloride, resulting in precipitation of said crystals in the resin solution.
As a result, if the crystals precipitate during the process before the solution is cast on the support piece, the filter becomes clogged. Also, if the crystals precipitate during the process after the solution is cast on the support piece, the transparency of the film decreases and foaming occurs around the crystalline nuclei, resulting in problems such as poor appearance, and the optical properties of a retardation film deteriorates obtained by using the film.
Also, methylene chloride moves slow in the resin and evaporates quickly from the surface of the resin solution, which results in the so-called dried thin-layer phenomenon (the phenomenon in which a dried thin-layer is formed on the surface) during the initial stage of drying. If the thickness of the polysulfone resin film being manufactured is 50 micrometers or more, the methylene chloride may be caught in the film-like product or between the film-like product and the support piece, resulting in foam in the film-like product.
Drying said resin film sufficiently while preventing this foaming involves a prolonged drying time, which leads to another problem, i.e. reduced productivity of said resin film.
Another reason for the prolonged drying time is that polysulfone resin forms a pseudo-linkage with methylene chloride, unlike polycarbonate resin, polyarylate resin and cellulose triacetate which are also made into films by means of the solution casting method using the same methylene chloride solution. The pseudo-linkage forms a complex of polysulfone and methylene chloride. Although methylene chloride is originally a solvent with a low boiling point, because of the formation of a complex, a high temperature or prolonged drying is required to sufficiently dry it, resulting in reduced productivity.